Electronic device and method of manufacturing thereof

ABSTRACT

The device has a carrier and an electric element. The carrier has a first and an opposed side and is provided with a connection layer, an intermediate layer and contact pads. The element is present at the first side and coupled to the connection layer. The element is at least partially encapsulated by an encapsulation that extends into isolation areas between patterns in the intermediate layer. A protective layer is present at the second side of the carrier, which covers an interface between the contact pads and the intermediate layer.

The invention relates to a method of manufacturing an electronic deviceprovided with an electric element and a carrier with a first side and anopposed second side, comprising the steps of:

-   -   providing the carrier comprising a patterned connection layer,        an intermediate layer and a continuous carrier layer, which        connection layer is present at the first side of the carrier,        said intermediate layer comprising an electrically conductive        material and having a pattern substantially corresponding to        that of the connection layer;    -   mounting the electric element to the carrier;    -   applying an electrically insulating material which extends to a        surface of the intermediate layer; and    -   removing the carrier layer at least partly and creating contact        pads for external contacting, which contact pads are connected        to the corresponding bond pads through interconnects in the        intermediate layer.

The invention also relates to an electronic device provided with anelectric element and an encapsulation of electrically insulatingmaterial, and a carrier with a first side facing the element and anopposed second side, said carrier comprising:

-   -   a patterned connection layer    -   contact pads for coupling to an external carrier or element, and    -   an intermediate layer of electrically conductive material, which        is patterned so as to create interconnects between the        interconnection layer and the contact pads, interconnects are        mutually isolated by isolation areas, the encapsulation        extending into the isolation areas.

Such a method and such a device are known from US-A 2001/0049156. Theknown device has a carrier with an intermediate layer of for instancecopper. The bond pads hereon are of a bilayer of nickel and gold. Thecarrier layer is for instance a polyimide or polyester tape that isattached to the intermediate layer with a layer of silicone adhesive.The intermediate layer is patterned with an etching method, wherein thebond pads act as etch mask. The etching is done such that that the sidefaces created in the intermediate layer have a concave profile. Thisenhances the adhesion between the intermediate layer and theelectrically insulating material that is subsequently provided to formthe encapsulation. After the etching an electric element is mounted onthe carrier. This element is in this case a semiconductor device that issubsequently encapsulated in the encapsulation. After the provision ofthe encapsulation, the carrier layer is completely removed. Then thecontact pads are provided in a protective flash treatment. Thistreatment results thereon that the portions of the intermediate layerthat remain after patterning thereof, are provided with a bilayer ofnickel and gold.

It is a disadvantage of the device of the invention, that the contactpads are located directly opposite to the bond pads. This isdisadvantageous as it is generally needed to do some rerouting in thecarrier in order to match the pitch of the bond pads with the pitch ofthe external board to which the contact pads are to be connected. Thedifferent pattern might be provided with the help of a photoresist mask,that is applied after removal of the carrier layer. However,photolithographic steps are undesired in assembly factory.

It is therefore a first object of the invention to provide a method ofthe kind mentioned in the opening paragraph, which results in a devicethat allows rerouting and/or interconnecting in the carrier and with agood adhesion of the carrier to the encapsulation without the risk ofdelamination during any further soldering steps.

This object is achieved in that

-   -   as the carrier a carrier with a carrier layer of electrically        conductive material is used;    -   the carrier layer is partly removed, so as to pattern it from        the second side of the carrier, therewith creating contact pads        for external contacting, which contact pads are connected to        corresponding patterns in the connection layer through        interconnects in the intermediate layer, and    -   a protective layer is provided at the second side of the carrier        which covers an interface between the intermediate layer and the        contact pads.

In the method of the invention, a specific carrier is used, whichcomprises a metal layers opposite to the intermediate layer. One ofthese layers acts as the carrier layer, and is patterned after theprovision of the electrically insulating material. The use of such acarrier is described in the non-prepublished applicationWO-IB03/01421(PHNL020327). It has the important advantage that thepatterns of the connection layer can be mechanically anchored in theelectrically insulating material, in that they extend laterally withrespect to the intermediate layer. It is furthermore the advantage thatthe pattern in the connection layer need not to be the same as thatprovided in the carrier layer.

However, it has been found that delamination problems may occur betweenthe intermediate layer and the contact pads. Such problems turn out tooccur in a further stage of processing when the device is soldered to anexternal board. This problem is now solved in that with a protectivelayer, that covers the interface between the intermediate layer and thecontact pads.

In a preferred embodiment, the carrier comprises a patterned maskinglayer at its second side. This masking layer will first function as anetch mask for patterning the carrier layer. Subsequently, it acts as anadhesion layer to solder needed for a connection to an external board.Suitable materials include bilayer of NiPd or NiAu or even NiPdAu. Byincluding the etch mask in the carrier, no additional photomask need tobe applied during the assembly process. A carrier with such an etch maskand its use is described in the non-prepublished application WOIB03/01299 (PHNL021100).

The constitution and the method of application of protective layer mayvary as desired in accordance with the further requirements of thedevice and with any technological possibilities of an assembly factory.Techniques such as spraying, dipping, coating and even vapour depositionare all applicable. This is particularly so, since the protective layeris applied before separation of the assembly into a plurality ofindividual devices. The main requirement is of course that the contactpads should not be damaged such that coupling to an external boardbecomes impossible.

The protective layer may both be electrically conductive andelectrically insulating. In case that the device is meant for use with alead-free solder, a temperature stability of this type of solder,currently about 260° C., is needed. A suitable electrically conductivelayer for use is combination with a carrier comprising Cu for thecontact pads and Al or an Al alloy for the intermediate layer is forinstance Sn. As this does not adhere to Al very well, the exposedsurface of the Al does not become solderable. An alternative might bethe use of Ni, but then measures are needed to prevent that a laterallyextending surface of the intermediate layer becomes solderable.

In case that the protective layer is an electrically insulating layer.It is then highly preferred to provide the protective layer in apatterned matter. This can be achieved in different ways.

In a first embodiment, use is made of the difference in surfaceproperties between the contact pads, particularly any adhesion layerthereon, and the other materials. This is for instance realized by thechoice of suitable material in a suitable solvent.

An alternative realization of this first embodiment is a two-stepmethod. In the first step, a first material is provided that adheres tothe contact pads selectively. As a result of which the surface of thecontact pad is modified to a desired surface property. In the secondstep, then the protective layer is provided, which will not adhere tothe material at the contact pad. The layer of first material may beremoved afterwards. However, if this layer is sufficiently thin and doesnot constitute a barrier for solder, this is not necessary. A suitablerealisation hereof, particularly in combination with an adhesion layerof for instance gold, is a thiol with modified end groups. Such a thiolwill form a monolayer on the gold surface. The exposed rear side of themonolayer can be given any desired surfacial properties.

In a second embodiment, use is made of a material that becomes fluid toa certain extent on heating. The pressure of the solder balls during thereflow processing for attachment to an external board is then sufficientto remove the protective layer locally. Examples of such materialsinclude acrylates and fatty acids, such as stearine acid.

The fatty acids are known from use in materials referred to as solderflux materials. Preferably, use is made of an organic solvent onapplication of these solder fluxes, such as alcohols. This tends to givea protection against any cleaning liquid conventionally used beforeplacement of the device on an external board.

The acrylates will melt on gentle heating, for instance to 100° C., andare subsequently cured. The use of these type of materials in assemblyprocesses is described in the non-prepublished application WO IB03/02292(PHNL020471), that is herein included by reference. The heating willmake the device moves downwards, therewith establishing a connectionbetween any solder ball and the contact pads. The subsequent raise ofthe temperature to higher temperatures, for instance during reflowprocessing or separately, will then cross-link the acrylate to make it alayer that is stable in temperature.

In a third embodiment, use is made of the cavity-like structure of thesecond side of the carrier. Then use is made of a material which ishardened or cured when present in the cavity, but not when present as athin film at the contact pads. A particular example hereof is a materialthat can be cured anaerobically. This is for instance an acrylate esterin a specifically chosen solvent, such as a acrylate ester in ahydroperoxide. A suitable example is a dimethylacrylate ester incumenehydroperoxide. Alternatively, a filling material is provided so asto fill the cavity-like structures, and subsequently a cleaning orpolishing step is done to bring the contact pads at the surface again.

In a fourth and preferred embodiment, the carrier is provided with amasking layer at its second side, through which masking layer first thecarrier layer is patterned into contact pads, as a result of whichpatterning process the masking layer has a larger surface area than thecorresponding contact pads, whereafter a positive photosensitivecomposition is applied at the second side of the carrier, that issubsequently patterned into irradiated and non-irradiated areas in aphotolithographical treatment in which the masking layer is used as amask, the irradiated areas being washed away. In this embodiment use ismade of the carrier as described in the non-prepublished application WOIB 03/01299 (PHNL021100). Since the etching of the carrier layer leadsthereto that the masking layer laterally extends to the carrier layer,the masking layer can be used in a subsequent step again. Positivephotosensitive compositions are known per se, as positive photoresists.Alternatively, they may be prepared by addition of a suitablephotochemical initiator to a suitable composition. Such a compositionmay include any desired material, which can be designed to have a goodadhesion to both the material of the contact pad and that of theintermediate layer. The material may even be an electrically conductivepolymer.

The electric element is preferably encapsulated in the electricallyinsulating material. However, this is not necessary per se. The elementmay be provided in any cavities at the first side of the carrier. Theelement may also be mounted at the second side of the carrier at the endof the process. In this case, the connection layer functions as aninterconnection layer parallel to the first side of the carrier. Ifencapsulated, the encapsulation may be present at the side faces of theelement only. The face facing away from the carrier is then availablefor the provision of any heat dissipating layer. This is particularlysuitable if the element is an integrated circuit or power device.

In a further method, use is made of a material that is applicable bydipping or immersing into a solution, particularly an aqueous solution.Suitable materials are those that are bound to the intermediate layer 12and/or the second metal layer 13 chemically. Examples hereof areinorganic materials such as CrO₂, TiO₂, ZrO₂, CeO₂. In addition, anorganic coating with a coefficient of thermal expansion similar tocopper may be applied. Suitable coatings would for instance besilicones, epoxgies and amides.

The electric element may very well be a semiconductor device, in thatthe resolution of the carrier is sufficient for both coupling withwirebonds and with flip chip. In the case that mounting of the electricelement in a flip-chip orientation is foreseen, the thickness of thecarrier layer is enlarged in comparison to the intermediate layer andthe connection layer. This allows an increase in the resolution of thepatterns defined in the connection layer and thus also in theintermediate layer, without loss of mechanical stability.

It is a second object of the invention to provide an electronic deviceof the kind mentioned in the opening paragraph, that allows reroutingand/or interconnecting in the carrier and has a good adhesion of thecarrier to the encapsulation without the risk of delamination during anysoldering steps exerted on the device.

This object is achieved in that a protective layer is present at thesecond side of the carrier so as to protect an interface between theintermediate layer and the contact pads. As indicated with reference tothe method of the invention, the protective layer solves the problem ofdelamination, and allows the use of a carrier wherein theinterconnection layer is mechanically anchored in the encapsulation.

It is preferred that the connection layer and the contact pads areembodied in copper, and a different material is used for theintermediate layer. Copper is a material with good conductivity that iswell known for use. Moreover, it can be used with a large variety ofbumps of different compositions, if necessary after provision of anadhesion layer or bilayer. Suitable materials for the intermediate layerinclude Fe—Ni, Fe—Cr—Ni, Al, alloys of Al, Ni, Cr, such asAl_(x)Si_(1-x), Al_(x)Cu_(1-x) and Al_(x)Ge_(1-x), with preferably0.5≦×≦0.99, and others.

In a preferred embodiment, at least a number of contact pads islaterally displaced with respect to the corresponding patterns in theconnection layer. The invention is particularly suitable for thisembodiment, in that the interface between the contact pads and theintermediate layer is herein is fully exposed at the second side of thecarrier. In the device known in the prior art, this is not the case, asthe NiAu will cover the intermediate layer fully. Also, the intermediatelayer therein comprises Cu. The adhesion of Cu to Ni is however not asproblematic as for instance the adhesion of Cu to Al.

It is advantageous herein, that the contact pads have a sufficientthickness. Therewith, the layer of the contact pads can be used ascarrier layer in accordance with the method of the invention. Moreover,the contact pads will have a surface that extends beyond the surface ofthe intermediate layer. The intermediate layer can thus be covered bythe protective layer, which thereto extends laterally.

In a further modification, the protective layer fills any cavity-likestructure between the contact pads. As a result hereof, the device hasat the second side of the carrier a surface that is substantiallyplanar. Proper observation of the second side of the carrier will showthat the contact pads arise as protrusions on a surface of theintermediate layer. With the cavity-like structure is then referred tothe space between the contact pads, which has in a cross-sectional viewthrough the contact pads the shape of a cavity.

In a preferred embodiment, the contact pads comprise a main layer and amasking layer, said masking layer being present at the second side ofthe carrier and having a larger surface area than the main layer. Theprotective layer is then substantially present between the masking layerand the intermediate layer, such that on perpendicular projection of theprotective layer on the masking layer there is a substantial overlap.This structure is the result of the fourth embodiment of the method ofthe invention. It is a suitable implementation that does not need anyphotolithographical masks in the assembly. An exposure to a light sourcesuffices. Furthermore, the embodiment has the advantage that the spacebetween the contact pads is not filled completely. This allows to copewith differences in thermal coefficients of expansion between thevarious layers more easily.

The element is preferably a semiconductor device that is at leastpartially encapsulated in the encapsulation.

These and other aspects of the invention will be further elucidated withreference to the figures, in which:

FIG. 1 is a diagrammatic cross-sectional view of a first embodiment ofthe electronic device;

FIG. 2 is a diagrammatic plan view of the first embodiment;

FIG. 3 is an enlargement of the cross-sectional view shown in FIG. 1

The figures are not drawn to scale. Like reference numerals refer tolike parts. Alternative embodiments are possible within the scope ofprotection of the appended claims.

FIG. 1 is a diagrammatic cross-sectional view of a first embodiment ofan electronic device 10. In this case, said electronic device is asemi-discrete semiconductor device with five contacts. This however isby no means essential. FIG. 2 is a diagrammatic plan view of the firstembodiment, wherein the line A-A indicates the cross-section of FIG. 1.The semiconductor device comprises a carrier 30 with a first metal layer11, an intermediate layer 12 and a second metal layer 13. In thisexample, the first and the second metal layer 11, 13 comprise Cu, andthe intermediate layer comprises Al_(0.99)Si_(0.01). Furthermore, thecarrier 30 comprises a first etch mask 14 and a second etch mask 17. Thefirst and the second etch mask 14, 17 each comprise an adhesive layer ofNiAu. The carrier 30 is patterned from the first side by means of thefirst etch mask 14, thereby forming apertures 15 and connectionconductors 31-35. For this purpose use is made of an etching processwherein first the first metal layer 11 is etched and subsequently theintermediate layer 12 is etched, thereby forming recesses 16 in the sidefaces of the connection conductors 31-35. Subsequently, thesemiconductor element 20 having connection regions 21 is connected tothe connection conductors 31-35 by connection means 22, in this casebumps of Au. For this purpose, use is made of a flip-chip technique.Subsequently the envelope 40 is provided, resulting in the formation ofa mechanical anchor since the envelope 40 extends into the recesses 16of the carrier. Mechanical anchoring can be achieved by any manner, inwhich one of the layers covered by the envelope 40 extends laterally,and this at least partially. These layers include the first etch mask14, the first metal layer 11 and the intermediate layer 12. Particularlyif the first metal layer 11 has sufficient thickness, in the order of 10μm or more, preferably at least 20 μm, the etching of this first metallayer 11 will give rise to a profile. Thus, the first etch mask 14 willextend laterally beyond the first metal layer 11, and this is sufficientfor the anchoring. The intermediate layer 12 can be chosen, inconnection herewith, to have any desired thickness and to be of anydesired material. Subsequently, the second metal layer 13 is patternedby means of the second etch mask 17. This is achieved by placing thedevice in an etch bath that selectively removes the second metal layer13 with respect to the intermediate layer as well as with respect to thesecond etch mask 17. Etching materials are known per se, and can also befound in the application WO IB 03/01299. The apertures 15 aresubsequently also used to separate the semiconductor devices 10. Thishas the additional advantage that the mechanical anchoring substantiallyencapsulates the connection conductors 31-35, i.e. not only at thelocation of the semiconductor element 20 but also beyond said element.The size of the semiconductor device 10 is, for example, approximately 1by 1 mm. The opening 15 has a width of, for example, 40-100 μm. Thethicknesses of the first metal layer 11, the intermediate layer 12 andthe second metal layer 13 were chosen to be, respectively, 30 μm, 40 μmand 30 μm. However, this may be chosen in accordance with therequirements of the application. The first metal layer 11 and theintermediate layer 12 could each have a thickness of about 20 μm,whereas the third metal layer has a thickness of 60 μm. According to theinvention, subsequently a protective layer 19 is applied. This is done,in this embodiment by providing a composition of dimethylacrylate esterin cumenehydroperoxide. This composition will harden out in the spacebetween the contact pads 31,32, but not at the surface. As aconsequence, the surface layers 17 are kept free of the protective layer19. Although the figure shows that the protective layer results in asubstantially planar surface, this is not necessary.

FIG. 3 shows a detail of FIG. 1, in which more precisely the form of thelayers is described as obtained in an embodiment of the invention. Theintermediate layer 12 is shown to have at the interface with the firstmetal layer 11 a smaller diameter than at the opposite side, resultingin the mechanical anchoring into the intermediate layer. Furthermore, itis clear that the etch mask 17 extends laterally beyond the third metallayer 13. Preferably, the etch mask is deformed in a controlled manner.This is for instance implemented by providing a block, for instance ofrubber at the surface of the etch mask 17. By pressing the somewhatdeformable block on the etch mask 17, it will push the edges of the etchmask 17 towards the intermediate layer 12. This prevents damage to theseetch masks afterwards.

1. An electronic device provided with an electric element and anencapsulation of electrically insulating material, and a carrier with afirst side facing the element and an opposed second side, said carriercomprising: a patterned connection layer, contact pads for coupling toan external carrier or an external element, and an intermediate layer ofelectrically conductive material, which is patterned so as to createinterconnects between the patterned connection layer and the contactpads, the interconnects are mutually isolated by isolation areas, theencapsulation extending into the isolation areas, wherein a protectivelayer is present at the second side of the carrier so as to protect aninterface between the intermediate layer and the contact pads, theprotective layer being in contact with the intermediate layer.
 2. Anelectronic device as claimed in claim 1, wherein the carrier includesbond pads at the first side of the carrier that are mechanicallyanchored in the encapsulation.
 3. An electronic device as claimed inclaim 2, wherein at least a number of the contact pads is laterallydisplaced with respect to the corresponding bond pads.
 4. An electronicdevice as claimed in claim 1, wherein the protective layer laterallyextends so as to cover the intermediate layer.
 5. An electronic deviceas claimed in claim 1, wherein the electric element is a semiconductordevice, that is at least partially encapsulated in the encapsulation. 6.An electronic device as claimed in claim 1, wherein the contact padsinclude a main layer and a masking layer, the masking layer beingpresent at the second side of the carrier, and wherein the protectivelayer is at least partially located between the masking layer and theintermediate layer.
 7. An electronic device as claimed in claim 1,wherein the protective layer is electrically conductive material, theprotective layer including at least one of Sn and Ni.
 8. An electronicdevice as claimed in claim 1, wherein the protective layer iselectrically insulating material.
 9. An electronic device comprising anelectric element; an encapsulation of electrically insulating material;a carrier having a first side facing the electric element and an opposedsecond side, said carrier including: a patterned connection layer,contact pads for coupling to an external carrier or an external element,the contact pads including a main layer and a masking layer, saidmasking layer being present at the second side of the carrier and havinga larger surface area than the main layer, and an intermediate layer ofelectrically conductive material that is patterned to createinterconnects between the patterned connection layer and the contactpads, the interconnects being mutually isolated by isolation areas, theencapsulation extending into the isolation areas; and a protective layerpresent at the second side of the carrier to protect an interfacebetween the intermediate layer and the contact pads, the protectivelayer being substantially present between the masking layer and theintermediate layer, such that on perpendicular projection of theprotective layer on the masking layer there is a substantial overlap.10. An electronic device as claimed in claim 9, wherein the carrierincludes bond pads located at the first side of the carrier, each of thebond pads having a corresponding one of the contact pads, and each ofthe bond pads being laterally displaced with respect to itscorresponding contact pad.
 11. An electronic device as claimed in claim9, wherein the protective layer is electrically conductive material, theprotective layer including at least one of Sn and Ni.
 12. An electronicdevice as claimed in claim 9, wherein the protective layer iselectrically insulating material.